(1) Field of the Invention
The present invention relates generally to filtration and, more particularly to an improved cross-flow membrane filtration process.
(2) Description of the Prior Art
The existence of many substances as solutions or mixtures created the need for processes to be developed to separate these solutions or mixtures. In this context, the need to purify, recover, isolate, and remove substances in process streams in chemical, pharmaceutical, food, petroleum, and wastewater applications has driven investigations in separation technology. In recent years, membrane-based cross-flow filtration has gained importance in many separation applications and, in some situations, competes with traditional separation technologies, such as distillation, absorption, and extraction.
The most common cross-flow membrane processes are microfiltration (MF), ultrafiltration (UF), and reverse osmosis (RO). The primary differences between these processes relates to the size and/or the molecular weight of the feed component to be separated. These processes are pressure-driven and are used for separation of macromolecules from a solvent, typically water. Examples of typical applications along with typical operating pressures are shown in Table 1 below.
For small transmembrane pressures, the membrane flux is proportional to pressure. The term xe2x80x9cmembrane fluxxe2x80x9d refers to the flow volume over time per unit area of membrane (ml/min/sq.meter). However, as the pressure of the flow into the membrane is increased, the flux reaches an asymptotic value beyond which a further increase in pressure does not contribute to any increase in the flux.
By varying the transmembrane pressure difference driving force and average pore diameter, the membrane in the MF, UF and RO processes may serve as a selective barrier by permitting certain components of a mixture to pass through while retaining others. This results in two phases, premeate and retained phases, each of which is enriched in one or more of the components of the mixture.
Some of the obstacles to the successful use of membrane separation processes are phenomena known as concentration polarization and fouling. Concentration polarization occurs when a concentration gradient of the retained components is formed at or near the membrane surface. Fouling is the deposition of material, referred to as foulant, on the membrane surface or in its pores, leading to a change in membrane behavior or even complete plugging of the membrane. These phenomena manifest themselves over time by increased operating pressure whereby the permeate flux reaches an asymptotic value beyond which further increases in operating time and pressure do not result in increased flux. The severity of the effects of these phenomena varies with the membrane type and the composition of the process stream.
Concentration polarization is a function of the hydrodynamic conditions in the membrane system. Membrane fouling is usually characterized as irreversible; however, when cross-flow membranes are used, the imposed stress of the cross-flow tends to shear the foulant layer. Hence, varying the fluid mechanics of a system is very important in maximizing the total capacity of a membrane module.
In the past, attempts have been made to manipulate the fluid hydrodynamics or membrane surface morphology to enhance transmembrane flux. However, these attempts have provided only limited success.
The effect of membrane surface modification by chemical and physical means has been investigated. The principle behind the idea of chemical modification is that it might reduce attractive forces or increase repulsive forces between the solute and membrane. However, this method has been found to have little effect on the behavior of suspended particles once a secondary cake has been established. Physical modification is achieved by using protuberances designed to induce instabilities in the bulk flow. Protrusions are placed on the membrane surface at defined intervals in such applications. This technique has the disadvantage of diminishing the useful surface area of the membrane. Additionally, this technique causes high axial pressure drops and can be difficult to scale up.
Techniques to modify fluid hydrodynamics of the bulk stream have also been investigated. Such modifications include the periodic induction of a pressure gradient on the feed stream. It has been found that the advantage of using pulsation overcomes the disadvantage of increased power consumption. However, the problems of energy dissipation and reduced cross-flow, which results in a lower net filtering capacity, remain.
Finally, fluid instabilities due to flow in curved ducts, known as Taylor and Dean flows, have been used to disturb the flux-limiting effects of concentration and fouling. The problems associated with this and other similar external devices are the high energy required to operate the devices, the difficulty to repair them, and the difficulty to scale them up.
Thus, there remains a need for a new and improved cross-flow membrane filtration process which provides a substantial improvement in flux-enhancement while, allowing automatic operation.
The present invention relates to a filtration system which reverses the feed flow in the cross-flow membrane filter to reduce the deleterious effects of concentration polarization and membrane fouling, thereby increasing the average transmembrane flux. The present invention changes the hydrodynamics of typical membrane systems by periodically reversing the direction of flow of the feed stream to the membrane. Periodic reversal of the direction of flow of the feed stream in the membrane module, while maintaining the cross-flow, has been found to keep the system in a hydrodynamically transient state and to prevent the formation of an undesirable stable boundary layer at the membrane surface. Therefore, the collection of particles in a gradient near the membrane surface and particle deposition on the membrane surface are slowed. Unlike backpulsing, in which the permeate stream is periodically forced back through the membrane module permeate outlet under the impetus of an induced pressure gradient, the feed flow direction itself is reversed from time to time through the filter without the need for an auxiliary pump, loss of pure permeate back through the filter or the possibility of stressing and breaking the membrane.
Accordingly, one aspect of the present invention is to provide a filtration system including: a feed supply for providing a feed solution; a feed pump connected to the feed supply; a cross-flow membrane filter connected downstream of the feed pump for separating the feed into a permeate and a retentate; and a valve manifold assembly located between the feed pump and the cross-flow membrane filter for selectively reversing the flow of the feed through the cross-flow membrane filter.
Another aspect of the present invention is to provide a filtration system including: a feed supply for providing a feed solution; a feed pump connected to the feed supply; a cross-flow membrane filter connected downstream of the feed pump for separating the feed into a permeate and a retentate, the cross-flow membrane filter including at least two membrane ports and at least two permeate outlets; and a valve manifold assembly located between the feed pump and the cross-flow membrane filter for selectively reversing the flow of the feed through the cross-flow membrane filter.
Still another aspect of the present invention is to provide a filtration system including: a feed supply for providing a feed solution; a feed pump connected to the feed supply; a cross-flow membrane filter connected downstream of the feed pump for separating the feed into a permeate and a retentate, the cross-flow membrane filter including at least two membrane ports and at least two permeate outlets; a valve manifold assembly located between the feed pump and the cross-flow membrane filter; and a control system for controlling the valve manifold to selectively reverse the flow of the feed through the cross-flow membrane filter.
These and other aspects of the present invention will become apparent to those skilled in the art after a reading of the following description of the preferred embodiment when considered with the drawings.